As said,
a) top off fluid,
b) replace cap
You did a), now do b). I worked on my rear axle bearings the other night and lost some brake fluid (not much), leading to exactly the symptoms you describe. In my case a) worked.

-Bill
'87 4Runner w/ '96 5VZ-FE, 'Red Chili II'
'97 Taco XtraCab 3RZ-FE, 'BlackBean'
TLCA # 13257, Rising Sun 4x4 Club Land Use Coordinator
"He who stops being better stops being good." -Oliver Cromwell